During the operation of electromechanical systems, such as an integrated drive generator (IDG) comprising a hydromechanical constant speed drive and a generator packaged as an integral unit, excessive heat can build up within the system that can result in costly damage or destruction of system components. In such systems, it is therefore advantageous to include a disconnect device for discontinuing operation of the system as soon as an overtemperature condition is encountered. Usually, this is accomplished by interrupting the flow of motive power by disconnecting a gear box output shaft from an input worm shaft of the IDG. Typically, an IDG contains an electrical actuator in the form of a solenoid which may be actuated to disconnect the shafts when circumstances require that the system be stopped.
In those systems that rely upon an electromechanical actuator to effect the desired mechanical movement that allows disconnection, there is always the problem that corrosion or the like may prevent the actuator from responding to a proper signal and, of course, it is always possible that a fault in the electrical circuit could prevent the disconnect signal from being generated and provided to the actuator in the first place.
Further, it has been found that, in the event of over heating, an electrical disconnect device may be rendered inoperable due to the melting of wires or other electrical components. Therefore, it is desirable to have a disconnect device in which automatic mechanical actuation is thermally triggered. Furthermore, it is desirable to provide such a thermal disconnect device while still maintaining the operability of the electrical solenoid.
It is highly undesirable that a disconnection should occur inadvertently since, at the very least, that would result in an undesired shut down of the system and the need to reset same. It is just as important that the system be fail-safe, that is, that there be provided some measure of redundancy or other means that prevents the disconnect system from being rendered inoperative for any of a variety of reasons.
In those instances utilizing thermally responsive material for effecting the mechanical movement, some difficulties may be encountered in terms of causing inadvertent disconnects as a result of unrestrained thermal growth as the temperature of the mechanism rises, although not to the danger point.
Still other approaches using eutectic material frequently require substantial clean up before the mechanism can be reset and used again. In some instances eutectic material may enter machined parts of the mechanism, thereby causing fouling and requiring substantial removal effort and/or replacement of parts.
Carlson, U.S. Pat. No. 3,212,613, assigned to the assignee of the present application, discloses a thermal disconnect device for an electromechanical system having a driving shaft, a driven shaft mechanically linked to the driving shaft by a splined connection and a thermally expandable material disposed in recesses in the driven shaft. When the temperature reaches a predetermined level, the thermally expandable material expands and exerts forces against a piston in turn disconnecting the splined connection between the shafts. For purposes of protecting the components of the electromechanical system, the temperature at which the material expands is selected to be lower than a temperature which would cause damage to the system should it continue to operate.
Swadley, U.S. Pat. No. 4,086,991, also assigned to the assignee of the present invention, discloses a thermally actuated disconnect coupling for driving and driven members interconnected by a coupling shaft having a splined connection. Within the driven member, a fusible element such as an eutectic pellet supports the coupling shaft against axial movement away from the driving shaft. When the temperature in the vicinity of the pellet reaches a predetermined level, the pellet melts, in turn permitting the coupling shaft to move away from the driving member so as to disconnect the driving and the driven members.
Dineen, U.S. Pat. No. 2,509,813 discloses an emergency disconnect device for driving and driven members interconnected by a splined coupling shaft. The splined coupling shaft includes an externally threaded portion which engages a threaded nut. When the driving and driven shafts are to be disconnected, a pin is moved into engagement with an external projection of the nut to in turn arrest rotation thereof so that the splined coupling shaft is moved out of engagement with the driven shaft. In alternative embodiments, the projection on the nut is engaged by a spring loaded shaft which is normally prevented from engaging the nut by a fusible pin that resists the urging of the spring to move the pin into engagement with the projection on the nut. When the temperature in the vicinity of the fusible pin rises to a certain level, the fusible pin melts and permits the spring to urge the pin into engagement with the projection on the nut.
Gaeckle, U.S. Pat. No. 4,271,947 discloses a thermal disconnect device which mechanically disconnects two aligned shafts in response to overheating of a eutectic metallic element. Under usual operating conditions (i.e., when an overtemperature condition is not present) the eutectic element opposes the urging of a series of spring washers to force the aligned shafts out of engagement with one another. When an overtemperature condition is encountered, however, the eutectic element melts, thereby causing the shafts to move out of engagement with one another under the influence of the spring washers.
Other types of devices utilizing thermally responsive elements include Barnham, et al., U.S. Pat. No. 1,571,182, Zahradnik, U.S. Pat. 2,127,319, Niederer, Sr., U.S. Pat. No. 4,016,722, Lamb, et al., U.S. Pat. No. 4,253,304, Gillette, U.S. Pat. No. 4,288,770 and U.K. Patent Application 2,036,202.
Johnson, et al., U.S. patent application Ser. No. 07/439,783, filed Nov. 21, 1989, entitled "Thermal Operator for Use in a Mechanical Disconnect or the Like " and assigned to the assignee of the present application discloses a prior art thermal disconnect device which can be retrofitted to a conventional solenoid disconnect device. A disconnect plunger is urged toward a series of external threads carried by a worm shaft which is in turn coupled by a jaw tooth clutch to an input or driving shaft. The plunger is restrained against movement into engagement with the external threads by a solenoid plunger which is normally disposed in a hole in the disconnect plunger. The solenoid plunger can be retracted out of the hole in the disconnect plunger to thereby allow the disconnect plunger to engage the external threads and thereby axially move the worm shaft away from the input shaft. This may be accomplished by actuating the solenoid or may occur in response to an overtemperature condition. When such a condition is encountered, a eutectic wax expands and displaces a piston within a housing containing the solenoid plunger. The piston carries an axially directed finger that in turn bears against an armature of the solenoid. As the piston is displaced, the armature moves axially and thereby retracts the solenoid plunger from the hole in the disconnect plunger.